1. Field of the Invention
The present invention relates to a method of driving a liquid crystal display device which is suitable for liquid crystal of quick response. In particular, the present invention relates to a passive matrix type liquid crystal display device performing a multiplex driving by a multiple line simultaneous selection method (see JP-A-6-27907, U.S. Pat. No. 5,262,881).
2. Prior Art
Hereinafter, in this specification, a scanning electrode is referred to as a row electrode or simply as a line, and a data electrode is referred to as a column electrode.
With the progress of the advanced information age, a need for information media of display has been more increasing. A liquid crystal display has advantages such as thin structure, light weight, low power consumption etc. and has good coordination with semiconductor technology, and accordingly it is expected to become more wide spread. With the spread of liquid crystal displays, there are requirements of a large-sized picture surface and high precision, and a search for a method of performing a large capacity display is beginning. Among them, a STN (super-twisted nematic) system has advantages that its manufacturing steps are simple and manufacturing can be performed at a low cost in comparison with a TFT (thin-film transistor) system.
A line-sequential multiplex driving method has conventionally been carried out in the STN system in order to achieve a large capacitance display. In this method, respective row electrodes are successively selected one by one and column electrodes are driven in correspondence with a pattern to be displayed, and the display of one screen is finished after all the row electrodes have been selected.
However, it has been known in the line-sequential driving method that it causes a problem called frame response with an increase in the display capacity. In the line-sequential driving method, a relatively large voltage is applied to a pixel in a selection time and a relatively small voltage is applied thereto in a non-selection time. The ratio of the voltages is generally increased with an increase in the number of lines (i.e., with an increase in high duty drive). Accordingly, it results that liquid crystal which has responded to an effective value of a voltage when the voltage ratio is small, now responds to an applied waveform. Thus, the frame response is a phenomenon in which the transmittance of the liquid crystal at an OFF time is increased since the amplitude of a selection pulse is large, the transmittance thereof at an ON time is decreased since the period of selection pulses is long and as a result, a reduction of the contrast ratio is caused.
Although there has been known a method of increasing the frame frequency by which the period of the selection pulse is shortened, to suppress the occurrence of the frame response, it has a serious drawback. Namely, when the frame frequency is increased, the frequency spectrum of the applied waveform becomes higher, and accordingly, nonuniformity of display is caused and power consumption is increased. Therefore, there exists an upper limit in the frame frequency in order to prevent the selection pulse width becoming too narrow.
A new driving method has recently been proposed to solve the problem without making the frequency spectrum higher, namely, a multiple line simultaneous selection method wherein a plurality of row electrodes (selection electrodes) are selected simultaneously. According to this method, a plurality of row electrodes are simultaneously selected and a display pattern in a column direction can independently be controlled. According to this method, the frame period can be shortened while maintaining the selection width constant. Namely, a high contrast ratio display while controlling the frame response can be achieved.
In the multiple line simultaneous selection method, when a plurality of row electrodes are simultaneously selected, a predetermined voltage pulse series is applied to the row electrodes. It is because it is necessary to apply pulse voltages having different polarities to the row electrodes in order to independently and simultaneously control the display pattern in the column direction. Pulses having polarities are applied by a plurality of times to the row electrodes and voltages in correspondence with data are applied to the column electrodes. In this way, effective voltages in response to ON and OFF are applied to respective pixels in total.
In this case, a group of selection pulse voltages applied to respective row electrodes can be expressed by a matrix of L rows and K columns (hereinafter, referred to as a selection matrix (A)). The selection pulse voltage series can be represented as mutually orthogonal vector groups and therefore, the matrix including these as column elements is an orthogonal matrix. Respective row vectors in the matrix are mutually orthogonal. The number of rows L corresponds to the number of simultaneously selected rows and each row corresponds to each line. For example, the element of the first line of the selection matrix (A) is applicable to line 1 among L selection lines. Further, voltages as selection pulses are applied in the order of the element of the first column, the element of the second column and so on.
With respect to the description of the selection matrix (A) in this specification, numeral 1 designates a positive selection pulse and numeral -1 designates a negative selection pulse. FIGS. 4(a), 4(b) and 4(c) show Hadamard's matrices as representative examples of the selection matrix (A). FIG. 4(a) shows that of 4 rows and 4 columns, FIG. 4(b) shows that of 8 rows and 8 columns and FIG. 4(c) shows that of 7 rows and 8 columns which is formed by removing the first column of that of 8 rows and 8 columns.
Voltage levels in correspondence with respective column elements of the matrices and a display pattern on the column electrodes are applied to the column electrodes. Namely, the column electrode voltage series are determined by the matrices determining the row electrode voltage series and the display pattern.
A sequence of voltage waveforms applied to the column electrodes is determined as follows. FIGS. 3(a), 3(b) and 3(c) are diagrams showing the concept. Explanation will be given with Hadamard's matrix of 4 rows and 4 columns as an example. The display data on a column electrode i and a column electrode j are as shown in FIG. 3(a). Column display patterns are designated by vectors (d) as shown in FIG. 3(b). Here, -1 of a column elements designates ON display and 1 thereof designates OFF display. When the row electrode voltages are successively applied to the row electrodes in the order of the columns of the matrix, the column electrode voltage levels become vectors (v) as shown in FIG. 3(b) and the waveforms are as shown in FIG. 3(c). In FIG. 3(c), arbitrary units are used for an ordinate axis and an abscissa axis.
In a case of partial line selection, it is preferable to apply voltages dispersibly in one display cycle in order to control the frame response of the liquid crystal display element. Specifically, for example, after applying the first element of the vector (v) corresponding to the first simultaneously selected row electrode group (hereinafter, referred to as the subgroup), the first element of the vector (v) corresponding to the second simultaneously selected row electrode group is applied, and the same sequence is carried out successively.
Thus, an actual voltage pulse sequence applied to the column electrodes is determined by how the voltage pulses are dispersed in one display cycle and which selection matrix (A) is selected to the respective simultaneously selected row electrode group.
Recently, a window pattern display has very frequently been used. When the window pattern display is effected, a phenomenon called crosstalk occurs, which is a problem in display.
Influence by the crosstalk is remarkable in displaying a bar-like image. Such phenomenon is described in JP-A-8-62574 and derives from a deformation in the driving waveform.
The other big problem is crosstalk in a display of intermediate tone. For systems of displaying an intermediate tone, there are a frame rate control (FRC) system, an amplitude modulation system, a combination thereof with a dither method and so on. However, the FRC system has widely been employed as the driving method for a liquid crystal display device. In this case, a combination of the FRC system and a technique for forming a phase difference in terms of space (i.e., between adjacent pixels) to cancel a flicker (i.e., a space modulation method) is frequently employed. When such a gray scale display is carried out, there is a case that the spatial frequency of an image becomes very high. The height of the spatial frequency causes the crosstalk to deteriorate the quality of the image. Similarly, when the dither system is employed, the spatial frequency is also increased. Thus, there existed the problem of crosstalk. Further, there is a problem of deterioration of image in a case that a dynamic image in a video display is to be displayed. In the video display, a spatially complicated display (i.e. a high spatial frequency) is often displayed unlike a basically geometrical display such as a window display. Accordingly, in a case of providing a video display in one window, there arose such problems that not only the quality of the video display itself was deteriorated by the crosstalk produced but also adverse influence was given to a peripheral window. In order to reduce the above-mentioned crosstalk, it is effective to lower the main component of frequency of a driving waveform into a more flat frequency region in the frequency characteristics of the liquid crystal display device, specifically, it is effective to introduce polarity inversion at a timing independent of the display frame. However, in the multiple line simultaneous selection method, the addressing method is fundamentally different from that in the conventional line-sequential driving method and accordingly, the introduction of the polarity inversion caused another special drawback of display whereby there was a big problem in achieving the reduction of the crosstalk and an improvement of the quality of display.
Further, in the multiple line simultaneous selection method, the plurality of data voltage levels are provided as described above and an actual waveform is determined by the display data and the orthogonal matrix used. Accordingly, there causes frequent transition in voltage levels, and this strongly influences the occurrence of the crosstalk. The formation of a waveform by the plurality of data voltage levels creates difficulty in controlling the crosstalk in the multiple line simultaneous selection method.
The present invention is to provide a driving method to overcome problems of the crosstalk and the quality of display in the multiple line simultaneous selection method.